The present invention relates in general to burnable absorbers, also referred to as burnable poisons, for nuclear reactors, and more particularly, to an annular burnable absorber rod for use in a nuclear reactor core of fuel assemblies and which annular burnable absorber rod is constructed to minimize the displacement of the moderator and coolant, i.e., water, flowing within the reactor core while controlling the reactivity and ultimately extending the operating life cycle of the fuel assemblies.
The process of nuclear fission involves the disintegration of fissionable nuclear fuel material into two or more fission products of lower mass number. Among other things, the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time, the fuel assembly with fissionable materials must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming, taking as much as six weeks, and costly in terms of lost power generation, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor. Such neutron-capturing elements are usually designated as burnable absorbers if they have a high probability or cross-section for absorbing neutrons while producing no new or additional neutrons or changing into new absorbers as a result of neutron absorption. During reactor operation, the burnable absorbers are progressively reduced in amount so that there is a compensation mode with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material, as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron cross-section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less absorber captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fissionable material and burnable absorber in carefully proportioned quantities, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity. Burnable absorbers which may be used include boron, gadolinium, samarium, europium, and the like, which upon the absorption of neutrons result in isotopes of sufficiently low neutron capture cross-section so as to be substantially transparent to neutrons.
The incorporation of burnable absorbers in fuel assemblies has thus been recognized in the nuclear field as an effective means of increasing fissionable material capacity and thereby extending reactor core life, for example, to eighteen months without the requirement for fissionable material replacement. Burnable absorbers are used either uniformly mixed with the fissionable material, i.e., distributed absorber, deposited as a coating on the exterior of nuclear fuel pellets containing fissionable material as disclosed in U.S. Pat. No. 3,427,222, or are placed as separate elements in the reactor core. Thus, the net reactivity of the reactor core is maintained relatively constant over the active life of a reactor core.
Although the use of burnable absorbers as separate elements in the reactor core has been known to extend the reactor core life and operating cycle, the use of such burnable absorbers in this manner has its limitations. For example, the use of a burnable absorber as a separate element, for example, in the form of rods, require a corresponding displacement and loss of moderator and coolant within the reactor core. This loss is undesirable as the heating of the nuclear reactor forces additional coolant out of the reactor core, and unless carefully controlled, can result in the nuclear reactor operating with a Positive Moderator Coefficient. To this end, there has been known the use of a burnable absorber in the form of a hollow stainless steel clad borosilicate glass tube containing approximately thirteen percent by weight of boron oxide as the burnable absorber material. However, these borosilicate glass tubes occupy a relatively large volume and therefore displace a corresponding large volume of coolant from the reactor core. This undesirable displacement of coolant is, in part, attributable to the relatively large volume occupied by the major inactive components of the borosilicate glass as compared with the relatively small volume occupied by the burnable absorber material.
Accordingly, it can be appreciated that there is an unsolved need for a burnable absorber rod which minimizes the displacement of coolant within a reactor core while controlling the reactivity and ultimately extending the operating life cycle of the fuel assemblies, in addition, to minimizing the material costs of such burnable absorber rods, as well as simplifying their construction.